The tank-ventilation valve of a tank-ventilation system of a motor vehicle, constituting a component that influences the exhaust gas, must be checked for its operativeness within the framework of the on-board diagnosis (OBD). From the German Published Patent Application No. 41 22 975 a method for ascertaining the operativeness of a tank-ventilation system can be gathered in which the signals for the volume flow of the tank-ventilation valve and the signals for the pressure differential between the tank interior and the ambient pressure are subjected to a cross-covariance analysis. The mentioned signals are formed by a high pass prior to the generation of the cross-covariance function, and the maximum or the averaged value of the cross-covariance function with respect to the product of the two input variables is calculated. A variance measure is generated for the signal of the volume flow through the tank-ventilation valve and a transmission coefficient calculated from the variance measure and the averaged value or maximum. If the transmission range is within a predefined range, the tank-ventilation system is considered operative. One advantage of this method is its independence from tank-pressure variations that are not due to changes in the volume flow through the tank-ventilation valve. Specifically, these are changes that are attributable to sudden gas formation in the tank, such as may be caused by sloshing fuel.
It is also known to implement the check of the tank-ventilation valve by controlling the tank-ventilation valve in idling in a sufficiently stable working point. Simultaneously, the change in the mixture composition and the change in the energy flow via the throttle valve, which represents the product of the air-mass flow via the throttle valve and the ignition-angle efficiency factor, are observed. Conclusions concerning an operative tank-ventilation valve are drawn as a function of this observation. It is problematic here that the check may be registered as a distraction by the driver of the vehicle, for example when a change in the rotational speed occurs.